Known hitherto as moving-bed type hearth furnaces are a rotary-hearth furnace, which is equipped with an outer circumferential wall, an inner circumferential wall, and an annular rotary hearth disposed between these walls, and a linear-hearth furnace, which is equipped with two side walls and a linear hearth disposed between these walls.
In general, the rotary hearth comprises an annular furnace body frame, a hearth heat insulator disposed on the furnace body frame, and a refractory disposed on the hearth heat insulator.
The rotary-hearth furnace having such a structure has conventionally been used, for example, for the heat treatment of metals, e.g., steel billets, or the incineration treatment of combustible wastes. In recent years, however, a method for producing reduced iron from agglomerate including a carbonaceous reducing material and an iron oxide-containing material using the rotary-hearth furnace is coming to be put to practical use. Furthermore, a method for producing high-purity granular metallic iron by heating agglomerate including a carbonaceous reducing material and an iron oxide-containing material in a reducing melting furnace, e.g., a rotary-hearth furnace, to reduce the iron oxide contained in the feed material while keeping the iron oxide in a solid state, thereafter further heating the yielded metallic iron to melt them, and aggregating the iron while separating the iron from the slag components, has recently been developed.
In the method for producing reduced iron or producing granular metallic iron using a rotary-hearth furnace, it has been necessary that, for evenly heating the fed agglomerate, the agglomerate should be dispersed and leveled over the whole hearth without fail. There also has been a problem that the powder or the like generated from the agglomerate sinters on the hearth and adheres thereto, resulting in damage to the screw type discharger, etc.
Prior-art techniques for overcoming such problems are explained below by reference to FIG. 8. FIG. 8 is a view illustrating one example of methods for adding an adhesion inhibitor to agglomerate, according to Patent Literature 1.
First, Patent Literature 1 relates to a method for operating a rotary hearth type reducing furnace 21 in which agglomerate P including a powdery metal oxide and a powdery carbonaceous material is heated to reduce the metal oxide and thereby produce reduced iron. In this method, an adhesion inhibitor Q is added to the agglomerate P before the adhesion inhibitor Q is added into the furnace 21.
In Patent Literature 1, however, in the case where the adhesion inhibitor Q is not evenly laid when the adhesion inhibitor Q is added beforehand to the agglomerate P, the quantity of heat transferred to the agglomerate P from an upper part of the hearth 22 is uneven due to differences in surface level in the width direction and circumferential direction of the hearth 22. As a result, even and high-quality granular metallic iron is not obtained, resulting in a decrease in product yield. In the case where agglomerate P is laid on an adhesion inhibitor Q with the state that the adhesion inhibitor Q has differences in surface level in the circumferential direction and width direction of the hearth 22, this method has a problem that when the reduced iron obtained by reducing the agglomerate P is scraped out, the reduced iron gets under the adhesion inhibitor Q, resulting in a large amount of reduced iron remaining unscraped. In addition, the problem that molten iron accumulates to inhibit the production still remains unsolved.
Next, Patent Literature 2 relates to a method for leveling a feed material for granular reduced iron, in which a leveling member is lowered so as to reduce the gap between the hearth and the spiral blade of the leveling member in response to fluctuations in the amount of the feed material introduced. In this method, the leveling member is raised or lowered so that the rate at which the gap between the hearth and the spiral blade is increased or reduced in accordance with the rate at which the feed amount increases or decreases or with the rate at which the average particle diameter fluctuates is adjusted.
However, in Patent Literature 2, there is no description concerning influences of differences in the property of feed material on the rotation speed of the leveling member and on a relationship between the blade and the shaft. When the rotation speed of the leveler and the relationship between the blade and the shaft are not suited for the material to be leveled, this leads to a trouble that the feed material pass through or are scattered.